This invention relates generally to angular positioning apparatus and methods, and more particularly to an apparatus and method for angularly positioning a payload, such as a camera, about multiple axes of orientation.
Many types of systems may use angular positioning, i.e., angular orientation, apparatus. For example, an antenna may be angularly positioned about multiple axes so that it can point to any orientation in three-dimensional space and can be moved to quickly and accurately track objects moving across the sky. A narrow beam detector may be precisely angularly positioned about multiple axes to detect a narrow beam transmitted from a source. A laser may be angularly positioned about multiple axes so that its beam is accurately aimed to desired locations.
In addition, it is desirable to be able to angularly position a payload on a moving platform, such as a camera which may be used to transmit images from a moving vehicle as it focuses on an object in space. The camera may keep objects within view by, for example, being positioned by an angular positioning apparatus that compensates for multi-dimensional movements of the vehicle. The angular positioning apparatus may do this by moving the camera in an appropriate direction relative to the motion between the vehicle and the object. If the vehicle bounces upward, the camera could be angled downward to keep the object within view.
Conventional positioning systems may only move a device in two directions, e.g., the pan and tilt directions, but few can do so accurately or quickly, and still fewer move a device in a third dimension, e.g., the roll direction. A pan direction is a direction corresponding to the direction of yaw for a vehicle in a horizontal plane. It is a side-to-side motion about a vertical axis through the vehicle, and may include a sweeping motion that describes an arc in a horizontal plane. A tilt direction is a direction corresponding to the direction of pitch. It is a rocking motion about a horizontal axis orthogonal to the direction of motion, such as the up-and-down vertical movement of the front of a ship, and may describe an arc in a vertical plane. A roll direction is a direction corresponding to an axis through the vehicle in the direction it is moving.
If the device being positioned is ground based, an angular positioning apparatus may only need to move the device in pan and tilt directions, i.e. about two rotational axes. However, if either the device or a target that the device is tracking is moving, the apparatus may have to rotate the device about three axes, such as pan, tilt, and roll axes.
Conventional angular positioning systems use various different hardware systems to move a payload. A first kind of system may have an arm that is pivotally attached, for example, to a horizontal plate. By pivoting the arm in a vertical plane about a point intersecting the plate, the end of the arm can trace a circular or semi-circular arc in the vertical plane. The endpoints of the arc may lie in the horizontal plate. This could move a payload attached to the end of the arm in a tilt direction. The horizontal plate could in turn be rotated about a line perpendicular to it and intersecting it at the pivot point. By rotating the plate about this line, the payload can trace a circular arc in a plane parallel to the horizontal plate. The payload could thus be moved in a pan direction. By pivoting the arm about the point and rotating the plate about the line, the end of the arm can trace a path anywhere along a spherical or semi-spherical surface whose center is the pivot point and whose radius is the length of the arm.
A second kind of conventional angular positioning system uses gimbals to position a payload. The gimbal may have a vertical support member perpendicularly mounted at one end to a plate lying in a horizontal plane and attached at its other end to the lowest point of, for example, a U-shaped member. The structure may resemble the letter "Y." A cross-member may be rotatably attached to the end points of the U-shaped member. A payload attached to the cross-member could be panned by rotating the vertical member about an axis parallel to the vertical member, and the payload could be tilted by rotating the cross member about the endpoints of the U-shaped member.
For several reasons the first kind of conventional angular positioning system cannot quickly and easily position its payload about multiple axes. The arm becomes increasingly unstable when, for example, it is pivoted so that its angle with respect to the horizontal plate decreases (i.e., as the payload gets closer to the horizontal plate). When this happens the payload's center of mass is no longer supported by the arm, and an increasing force must be applied to either move or support the payload. Thus, any force used to tilt the arm becomes non-uniform and depends on the amount the arm is already tilted. This non-uniform force makes it difficult to quickly and accurately move the arm, and thus the payload, to a desired position. The system may also need locking mechanisms to maintain the arm at this otherwise unstable position. Complex motors and locking mechanisms also typically make these conventional systems expensive.
These difficulties are exacerbated when, for example, a payload, such as a camera, is to be positioned along a roll axis, as when the camera is mounted on a moving vehicle. To compensate for any roll of the vehicle the conventional positioning system would have to rotate the entire horizontal plate with the attached arm along an axis parallel to the vehicle's line of movement. The structure of this system is unwieldy, often heavy, and typically unstable. For reasons similar to those discussed above, the system would require a large force, having a torque dependent on the position of the payload, to rotate the horizontal plate and arm in a roll direction. The system would also require locking mechanisms, in addition to those needed to maintain the payload in a tilt direction, to maintain the payload in otherwise unstable positions along the roll axis.
The second kind of angular positioning system also does not easily rotate any attached payload about more than two axes. For example, the entire system may be attached to another gimbal that rotates the system about a roll axis. Like the first kind of conventional system, this two-layered system may be unwieldy, heavy, and unstable, and a large, non-uniform force may be required to move the payload as the payload's center of mass is moved about a roll axis. The system may also require locking mechanisms to maintain the payload in an otherwise unstable position. Therefore, it is desirable to angularly position and maintain a payload about multiple axes extending in a desired direction using a uniform and minimal force.
Another problem with conventional angular positioning systems is that they may require a large amount of clear space around them for a payload attached to the end of the arm to be moved to the limit of its angular, i.e., pan, tilt, and roll, range. For example, in the first type of angular positioning system the arm and any mounted payload may define an arc whose length is equal to the combined length of the arm and mounted payload. Thus, for example, to sweep through a 180.degree. arc in a tilt direction (e.g., in a vertical plane), the angular positioning system may need a clear distance equal to the length of this arc both in front of it and behind it. This requirement makes it difficult to move the positioning system and payload into tight areas. For similar reasons, the second kind of angular positioning system may also require a clear space around it. It is desirable to angularly position a payload using a minimum clearance so that the payload can be positioned in tight areas. Thus, there is a need for a system and method for angularly positioning a payload which avoids these and other problems of known systems and methods, and it is to this end that the present invention is directed.